Prediction of the Interface Deformation of Ultrasonic Spot Welding of Multilayer Metal Foils Considering Energy Gradient

The ultrasonic spot welding (USW) is widely used in the joining of multilayer Cu or Al tabs in the lithium-ion battery. Due to the high-frequency vibration of the sonotrode and various deformation in each interface, the friction behavior is complex which makes it difficult to simulate the welding process of multilayer specimens. In this paper, an efficient and accurate finite element model (FEM) was proposed via introducing the interface heat flux to equivalent the heat generation by the friction. The total heat flux was determined by the heat transfer analysis and the proportion of each interface was determined based on the analysis of the friction behavior. Then, the FEM was validated by comparing the simulated temperature and deformation with experimental results. Finally, the FEM was applied to simulate the USW process of four, five, and ten layers of copper and aluminum foils in order to characterize the gradient of the ultrasonic energy. It was found that the heat generation concentrated in middle interfaces was 65% of the total in the welding of four-layer copper foils. The heat generation was mainly related to the welding parameters and the proportion was related to the size of tips and the structure of specimens. The plastic strain varied in specimens because of the gradient of the input energy. It was most obvious in the welding of 10-layer aluminum foils that the maximum equivalent plastic strain (PEEQ) in the fifth interface was 92.9% smaller than the top interface.